1. Technical Field
The present invention relates to methods of manufacturing the shaft unit of an electrically powered motor.
2. Description of the Related Art
Hard-disk drives conventionally are equipped with a spindle motor (also referred to simply as “motor” hereinafter) that rotationally drives the recording disk(s). Such disk-drive motors are provided with a threaded hole into which a clamping member screws for fixing the recording-disk(s) to the motor's rotor hub, which includes a shaft. In some implementations, a shaft-penetrating configuration for the threaded hole is adopted, with the hole being formed heading into the shaft interior through the top endface of the rotor hub.
In implementations in which the threaded hole (or a hole that is linked with the threaded hole) penetrates the shaft, the interior of the motor and the space in which the recording disk(s) is housed communicate via the threaded hole. As a consequence, the space around the recording disk(s), which should stay clean, ends up being invaded by fine particulate matter generated in the motor. This is of particular concern in motors furnished with a fluid dynamic-pressure bearing, in which case the rotor hub is rotationally supported by oil filling a micro-gap in between the shaft and a sleeve for the bearing. In such configurations, the lower end of the shaft is disposed within the oil, which consequently runs the risk that the oil will travel along the inner surface of the threaded hole and invade the recording-disk area of the motor.
To address such problems with rotor hubs of this sort, in which a through-hole is formed in the shaft, technology whereby the through-hole is sealed to keep clean the space in which the recording disk(s) is housed has been proposed. For example, in the spindle-motor rotor hub art, a spherical elastic member of a diameter that is slightly larger than the through-hole diameter is inserted into the through-hole through the lower-end side of the shaft to plug the through-hole.
With regard to sealing the through-hole, in implementations in which, in order further to improve the adherence between the through-hole sealing member and the inner surface of the through-hole, a metal fitting, for example, is press-fitted into the through-hole. In this case, there is a likelihood that the shaft, whose diametric span is not substantial, will end up warping, thereby depriving the structure of the tolerances provided by the high precision under which the shaft was machined.